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Context and Summary
Advanced LIGO
Gravitational waves offer a remarkable opportunity to see the universe from
a new perspective, providing access to astrophysical insights that are available
in no other way. The initial LIGO gravitational wave detectors have started
observations, and are already yielding data that are being interpreted to
establish new upper limits on gravitational-wave flux.
The sensitivity of the initial LIGO instruments is such that it is perfectly
possible that discoveries will be made. If they succeed, there will be a strong
demand from the community to improve the sensitivity allowing more astrophysical
information to be recovered from the signals. If no discovery is made, there will
be no lesser urgency to improve the sensitivity of the instrument to the point
where there is a general consensus that gravitational waves will be detected often
and with a good signal-to-noise ratio. The development of the next generation of
instrument must be pursued aggressively to make the transition from the initial to
the Advanced detector in a timely way - after the complete science run of the initial
detector, but as quickly as possible thereafter.
The Advanced LIGO detector upgrade meets these requirements for an instrument that
will establish gravitational-wave astronomy. It is more than ten times more sensitive,
and over a much broader frequency band, than initial LIGO. It can see a volume of space
more than a thousand times greater than initial LIGO, and extends the range of compact
masses that can be observed at a fixed signal strength by a factor of four or more.
This proposal to build Advanced LIGO has grown out of the LIGO Scientific Collaboration
and has broad support both nationally and internationally from that community. A closely
coordinated community R&D program, exploring the instrument science and building and
testing prototype subsystem elements, has brought the design to a highly refined state.
The LIGO Laboratory will lead and coordinate the fabrication and construction of the
instruments, with the continued strong participation of the community.
The joint United Kingdom/German
GEO Project has received funding
to provide a capital investment in this construction project.
They are applying these resources to providing the
suspension subsystem, including suspension assemblies, their controls, and installation
and commissioning. The German proposal is also making a capital contribution
to Advanced LIGO. It is for the design and fabrication of the pre-stabilized laser subsystem.
The GEO Project is a full partner in Advanced LIGO, participating at all levels
in the effort.
Advanced LIGO can lead the gravitational-wave field to maturity.
The LIGO Mission
From its outset, LIGO has been approved by the National Science Foundation to directly
observe gravitational waves from cosmic sources, and to open the field of gravitational
wave astronomy. The program and mission of the LIGO Laboratory is to:
- observe gravitational wave sources,
- develop advanced detectors that approach and exploit the facility limits on
interferometer performance,
- operate the LIGO facilities to support the national and international scientific
community,
- provide data archiving for the LIGO data and contribute computational resources
for the analysis of data,
- develop the software infrastructure for data analysis and participate in the
search and analysis,
- and support scientific education and public outreach related to gravitational
wave astronomy.
LIGO is envisioned as a new capability contained in a set of facilities and not as a single
experiment. The LIGO construction project has provided the facilities that support the
scientific instrumentation, and the initial set of laser interferometers to be used in the
first LIGO scientific observation periods.
The facilities include the buildings and vacuum systems at the two observatory sites. The
two observatories are located at Hanford, Washington and Livingston, Louisiana. The performance
requirements on the LIGO facilities were intended to accommodate the initial interferometers
and future interferometer upgrades and replacements, and possible additional interferometers
with complementary capabilities. The requirements on the LIGO facilities were intended to
permit future interferometers to reach levels of sensitivity approaching the ultimate limits
of ground-based interferometers, limited by reasonable practical constraints on a large
facility at a specific site.
This proposal is for the second generation of instruments to be installed in the LIGO
infrastructure, and is expected to bring the science of gravitational radiation from a
discovery mode to a mode of astrophysical observation.
LIGO Detector Scientific Goals
The scientific program for LIGO is both to test relativistic gravitation and to open the
field of gravitational wave astrophysics. More precise tests of General Relativity (and
competing theories) will be made. LIGO will enable the establishment of a brand new field
of astronomy, using a completely new information carrier: the gravitational field.
Initial LIGO represents an advance over all previous searches of two or three orders of
magnitude in sensitivity and in bandwidth. Its reach is such that, for the first time,
foreseeable signals due to neutron-star binary “inspirals” from the Virgo Cluster (15 Mpc
distant) would be detectable. At this level of sensitivity, it is plausible, though not
certain, that the first observations of gravitational waves will be made. If signals are
not observed with initial LIGO, we will have set challenging upper limits on gravitational
wave flux, far beyond the capability of any previously existing technology.
The Advanced LIGO interferometers proposed here promise an improvement over initial LIGO
in the limiting sensitivity by more than a factor of 10 over the entire initial LIGO frequency
band. It also increases the bandwidth of the instrument to lower frequencies (from ~40 Hz to
~10 Hz) and allows high-frequency operation due to its tunability. This translates into an
enhanced physics reach that during its first several hours of operation will exceed the
integrated observations of the 1 year LIGO Science Run. These improvements will enable the
next generation of interferometers to study sources not accessible to initial LIGO, and to
extract detailed astrophysical information. For example, the Advanced LIGO detectors will
be able to see inspiraling binaries made up of two 1.4 M neutron stars to a distance of
300 Mpc, some 15x further than the initial LIGO, and giving an event rate some 3000x greater.
Neutron star - black hole (BH) binaries will be visible to 650 Mpc; and coalescing BH+BH
systems will be visible to cosmological distance, to z=0.4.
The existence of gravitational waves is a crucial prediction of the General Theory of Relativity,
so far unverified by direct observation. Although the existence of gravitational radiation is
not a unique property of General Relativity, that theory makes a number of unambiguous predictions
about the character of gravitational radiation. These can be verified by observations with LIGO.
These include probes of strong-field gravity associated with black holes, high-order post-Newtonian
effects in inspiraling binaries, the spin character of the radiation field, and the wave
propagation speed.
The gravitational wave "sky" is entirely unexplored. Since many prospective gravitational wave
sources have no corresponding electromagnetic signature (e.g., black hole interactions), there
are good reasons to believe that the gravitational-wave sky will be substantially different
from the electromagnetic one. Mapping the gravitational-wave sky will provide an understanding
of the universe in a way that electromagnetic observations cannot. As a new field of astrophysics
it is quite likely that gravitational wave observations will uncover new classes of sources
not anticipated in our current thinking.
Detector Design Fundamentals
The effect of a propagating gravitational wave is to deform space in a quadrupolar form.
The effect alternately elongates space in one direction while compressing space in an
orthogonal direction and vice versa, with the frequency of the gravitational wave. A
Michelson interferometer operating between freely suspended masses is ideally suited to
detect these antisymmetric distortions of space induced by the gravitational waves; the
strains are converted into changes in light intensity and consequently to electrical signals
via photodetectors.
Limitations to the sensitivity come from two sources: extraneous forces on the test masses,
and a limited ability to sense the response of the masses to the gravitational wave strain.
The thermally excited motion of the test mass and the suspension is a fundamental limitation,
intrinsic to the way in which the measurement is performed; this influence is managed through
the selection of low-mechanical-loss materials and designs which capitalize on them. Seismic
motion causes forces on the mirrors due to the direct coupling through the isolation and
suspension system, a technical noise source which is minimized through design; and due to
the time-varying mass distribution near the mass (the Newtonian background).
Sensing limitations arise most fundamentally due to the statistical nature of the laser
light used in the interferometry, and the momentum transferred to the test masses by the
photons (linking the sensing and stochastic noise limitations to sensitivity). Technical
noise sources that limit the ability to sense include frequency noise and intensity
fluctuations in the laser light. Scattered light, which adds random phase fluctuations
to the light, can also mask gravitational signals.
In the limit, valid for LIGO, that the instrument is short compared with the gravitational
wavelength, longer arms give larger signals. In contrast, most competing noise sources
remain constant with length; this motivates the 4km baseline of the Observatories. More
generally, the scientific capability of LIGO is defined within the limits imposed by the
physical settings of the interferometers and by the facility design, by the design of the
initial detectors and ultimately by future interferometers designed to progressively exploit
the facility capabilities.
Although the rates for gravitational wave sources have large uncertainty, an improvement
in strain sensitivity linearly improves the distance searched for detectable sources. This
increases the detection rate by the cube of the sensitivity improvement.
The Observatories
LIGO Facility Scientific Capability
The LIGO facility design envisaged a progression of increasingly sensitive interferometers
capable of extending the physics reach of the observatories. In the design of the
observatories, LIGO incorporated critical design features into its facilities in order
to optimize LIGO’s ultimate performance capabilities. These features include a building
foundation and infrastructure which provides a clean, quiet environment for the instruments;
a 4km long "L" ultra-high vacuum beam tube system that brings scattered light and index
fluctuations due to residual gas to a negligible level; and a system of large vacuum chambers
and pumping subsystems capable of providing a flexible envelope for a wide range of detector
designs, and delivering a vacuum quality that complements the beam tube subsystem. Advanced
LIGO requires no changes in this infrastructure to meet its scientific goals.
The LIGO Observatories
LIGO Hanford Observatory (LHO), located on the U.S. Department of Energy Hanford site
in eastern Washington, comprises 5 major experimental halls for the interferometer spread
over 5 miles. 1.2-m diameter ultrahigh vacuum tubing connects these halls. Three support
buildings house laboratories, offices, and an amphitheater, and two additional buildings
are associated with maintenance and operations. Approximately 90,000 square feet of this
space is under tight environmental control to minimize contamination of sensitive equipment.
The physical plant has been designed to provide a low vibration environment similar to the
surrounding undeveloped shrub-steppe environment.
LHO houses two interferometers with arm lengths of 4 km and 2 km. The 4-km equipment is
installed in vacuum chambers in the corner station and the two end stations on each arm.
The 2-km equipment uses vacuum chambers in the corner station and the two mid-stations
situated halfway down each arm. The two interferometers share 2 km of beam tube along
each arm. The beam tube can eventually accommodate up to 5 interferometer beams and the
current station buildings can accommodate up to 3 interferometers to accommodate future
growth.
Figure 1 LIGO Hanford Observatory (LHO) in aerial view.
The 4-km interferometer arms are shown with the 5 main buildings
along the orthogonal arm layout
Figure 2 LIGO Livingston Observatory (LLO) corner region in aerial view.
The LIGO Livingston Observatory, located in pine forests between Baton Rouge
and New Orleans, Louisiana, is the site of a single 4-km laser interferometer
gravitational wave detector. Construction of its physical facilities, scaled
to accommodate one interferometer, is complete. The beam tube dimensions are
identical to those at LHO.
Initial LIGO
Status of the LIGO Construction Project
The NSF Cooperative Agreement of May 1992 initiated LIGO Construction and
Construction Related Research and Development. The Project schedule and cost
estimates were reviewed by the NSF during September 1994 and presented to the
National Science Board in November 1994. The total funding established by the
Board for Construction and Construction Related R&D were $272.1 million and
$20.0 million, respectively. In addition, the NSF provided $68.7 million for
Operations through September 30, 2001 covering the period of Installation and
Commissioning. The LIGO construction effort is complete, on cost
and close to schedule.
Initial LIGO Detector Commissioning
Installation and commissioning is complete for the three interferometers.
The instruments have shown a steady
improvement in sensitivity, at all frequencies in the planned observation band,
during the commissioning process. The present sensitivity is shown in Figure 3.
All of the instruments have met the sensitivity goal for initial LIGO of an
RMS strain sensitivity of 10-21 in a 100 Hz band.
The present limits to performance are
understood through a combination of measurement and system modeling.
Figure 3 This figure shows the progression in the strain sensitivity
as a function of frequency. The Goal curve exceeds the requirement by about
a factor of three.
Future improvements to exceed the design sensitivity will involve some modest
increases in laser power, and modifications to the
electronics and optimization of control systems and filters.
It is anticipated that this series of improvements in performance
will largely take after the present science run,
starting in 2007 or 2008.
The LIGO Science Runs
The observatories are presently observing in the fifth science run (S5).
An integrated year of data will be collected, and it is anticipated that
the run will end in 2007, taking into account duty cycle, maintenance,
and tuning breaks. The LIGO Scientific Collaboration (LSC, please see
below) analysis groups are undertaking the search for chirp signals from
binary inspirals, periodic signals from neutron stars, burst signals from
e.g., supernovae and gamma ray bursts, and from a possible stochastic
noise background. The data analysis pipelines utilize a variety of
sophisticated filtering techniques - templates, time-frequency analysis,
inter-interferometer correlations, and use of the auxiliary and
environmental data channels, as examples. Data are also being correlated
with relevant optical data and, in the case of supernovae, with neutrino
signals. Results from the analysis of S1 data, providing new upper limits
to the gravitational wave flux, are being prepared for publication.
LIGO Scientific Collaboration
A fundamental goal of LIGO has been to become a true national facility available to the
scientific community. In order to accomplish this, LIGO has broadened the participation
to include the community of scientists interested in participating in the LIGO research
program by creating the LIGO Scientific Collaboration (LSC). There are now some 470 members,
from 30 institutions in both the US and internationally. The LSC consists of both LIGO
Laboratory scientists and those from collaborating groups. The LSC is organized so as to
provide "equal scientific opportunity" to all members whether they are from within LIGO
Laboratory or the broader LSC. It is growing steadily and will remain open to new members
over the coming years. It is worth noting that the LSC has significant international
participation, including collaborating groups from India, Russia, Germany, U.K, Japan and
Australia. The international partners are involved in all aspects of the LIGO research
program.
The full LSC collaboration meets twice yearly in an extended meeting, and various working
groups meet more frequently. The LSC has produced White Papers that outline the plans for
technical development of LIGO and for science data analysis. A publication policy and a
conference committee are active, as well as the other functions necessary to make it a
"full service" organization.
The Advanced LIGO design, both in basic conception and in the detailed R&D, is very much
a product of the LSC (with a strong LIGO Laboratory element). The technical working groups
have been and continue to be central to the advancement of the design, and this proposal
is made with the strong support of the many participating institutions in the LSC.
In addition, LIGO has been organized such that the search for astrophysical signals and
interpretations will be performed through the LSC. The collaboration members committed to
"LIGO I", the initial LIGO detector science runs, will be responsible for the science in
this beginning phase of observation. This group in LIGO is well defined and presently
consists of 111 LIGO Laboratory scientists and 134 scientists from collaborating institutions.
Preparation tasks for the runs are organized within the LSC, LSC members participate in
the data taking runs, and the analysis of the data is coordinated through the LSC proposal
driven process.
LIGO is available to all interested researchers through participation in the LSC, an open
organization. To join, a research group defines a research program with the LIGO Laboratory
through the creation of a Memorandum of Understanding (MOU) and relevant attachments. The
group then presents its program to the LSC. When the group is accepted into the LSC it
becomes a full scientific partner in LIGO.
Exploring the LIGO Capability: Advanced LIGO
As noted above, LIGO is designed to evolve and to support improvements in gravitational
wave detectors. A natural time for an upgrade to the instruments can be foreseen once
the initial LIGO goal of one year of integrated observation has been met. The initial
LIGO infrastructure is well designed to deliver its planned performance, and small
advances in sensitivity at higher frequencies may be possible with e.g., modest increases
in the input laser power. However, a large improvement will require an upgrade of the
entire detector in a coordinated fashion. The considerable research and development in
the Laboratory and the greater community that has taken place since the initial LIGO
interferometer design was frozen enables this improvement. The Advanced LIGO instrument
fulfills our requirements of a significant step forward in sensitivity, and can be
delivered on a time scale that meshes with the end of the Initial LIGO observation plan.
Overview of Advanced LIGO
The sensitivity goals for the Advanced LIGO detector systems are chosen to enable
the advance from plausible detection to likely detection and rich observational
studies of sources. These sensitivity goals require an instrument limited only by
fundamental noise sources over a very wide frequency range. To achieve this sensitivity,
almost every aspect of the interferometer must be revised from the initial LIGO design.
The system briefly described below is the reference concept that is the basis for
structuring the R&D program and the detailed studies of system tradeoffs performed
as R&D results define the feasible parameters. A more complete description of the
proposed detector, organized by subsystem, is found in the Advanced LIGO
Reference Design.
The basic optical configuration is a power-recycled and signal-recycled Michelson
interferometer with Fabry-Perot "transducers" in the arms. Using the initial LIGO design
as a point of departure, this requires the addition of a signal-recycling mirror at
the output "dark" port, and changes in the RF modulation and control systems. This
additional mirror allows the gravitational wave induced sidebands to be stored or
extracted (depending upon the state of "resonance" of the signal recycling cavity),
and leads to a tailoring of the interferometer response according to the character
of a source (or specific frequency in the case of a fixed-frequency source). The
planned upgrade includes the three LIGO interferometers, allowing e.g., one
interferometer at Hanford and the interferometer at Livingston to be tuned to
be broadband, and the second interferometer at Hanford to be used as a higher-frequency
narrowband detector.
To improve the quantum-limited sensitivity, the laser power is increased from the
initial LIGO value of 10 W to ~200 W. The conditioning of the laser light follows
initial LIGO closely, with a ring-cavity mode cleaner and reflective mode-matching
telescope.
Whereas initial LIGO uses 25-cm, 11-kg, fused-silica test masses, the test mass
optics for Advanced LIGO are larger in diameter (~34 cm) to reduce thermal noise
contributions and more massive (~40 kg) to keep the radiation pressure noise to a
level comparable to the suspension thermal noise; fused silica is the baseline material
for the test masses. Compensation of the thermal lensing in the test mass optics
(due to absorption in the substrate and coatings) is added to handle the much-increased
power - of the order of 1 MW in the arm cavities.
The test mass is suspended by fused silica fibers, in contrast to the steel wire
sling suspensions used in initial LIGO. The resulting suspension thermal noise is
anticipated to be less than the radiation pressure noise (in broad-band observation mode)
and to be comparable to the Newtonian background ("gravity gradient" noise) at 10 Hz.
The complete suspension has four pendulum stages, contributing to the seismic isolation
and providing multiple points for actuation.
The seismic isolation system is built on the initial LIGO piers and support tubes but
otherwise is a complete replacement, required to bring the seismic cutoff frequency
from 40 Hz (for initial LIGO) to 10 Hz. RMS motions (frequencies less than 10 Hz) are
reduced by active servo techniques. The result is to render the seismic noise negligible
at all observing frequencies. Through the combination of the seismic isolation and
suspension systems, the required control forces on the test masses will be reduced
by many orders of magnitude in comparison with initial LIGO, reducing also the probability
of non-Gaussian noise in the test mass.
The overall performance of Advanced LIGO is dominated at most frequencies by the quantum
noise of sensing the position of the test masses, with a contribution at mid-frequencies
from the internal thermal noise of the test masses. This design, with modest enhancements
after it enters scientific use, should take this interferometer architecture to its technical
endpoint; it is as sensitive as one can make an interferometer based on familiar technology:
a Fabry-Perot Michelson configuration with external optical readout using room temperature
transmissive optics. Further advances will come from R&D that is just beginning, such as
the exploration of cryogenic optics and suspensions, purely reflective optics, and a change
in the readout to one which fully exploits our nascent understanding of the quantum nature
of the measurement (e.g., quantum non-demolition speed meters). These later developments
will be timely for instruments to be developed in the second decade of this century.
Advanced LIGO R&D
Through 2007, most LIGO Laboratory detector R&D is being directed at the
challenges posed by the Advanced LIGO design. This R&D program is a significant part
of the larger R&D program of the LIGO Scientific Collaboration. The LSC program has
been developed in a collaborative manner and is coordinated through the LSC Working
Groups and by the LIGO Laboratory.
The R&D program currently underway, supported by the NSF under cooperative agreement
PHY-0107417, is designed to support this construction proposal. This proposal requests
that construction funding commence at a time when the major R&D issues are satisfactorily
resolved, and the subsystems are ready for fabrication.
The activities carried out prior to construction funding include small-scale fundamental
research motivated by Advanced LIGO. Examples of this are studies of core optic substrate
absorption, measurement of mechanical losses in suspension fibers, and studies of candidate
photodetectors for the gravitational wave readout. The R&D program also includes large-scale
prototypes of subsystems such as full-scale seismic isolation systems, full-scale suspensions,
and full-scale core optics. In order to carry out this research, in most cases these
components must be fully engineered to the realistic requirements and configuration of
Advanced LIGO. In order to study the performance and control of a suspension subsystem,
for example, the prototype studied must represent the Advanced LIGO design down to details
such as suspension fiber material, bonding technique, as well as control electronics design,
component selection, and physical layout. This kind of rigorous full-scale development
program is needed to reduce risks prior to defining and committing to a construction project
and to minimize the time between installation and the start of observations at the design
sensitivity.
For more detailed information on the Advanced LIGO
techical design please see the
Advanced LIGO Reference Design document.
Advanced LIGO Schedule and Cost
The Advanced LIGO fabrication and construction schedule grows out of the tightly coordinated
R&D program currently underway. The objectives in establishing the schedule are to
- Allow the initial LIGO instruments to be fully exploited, and in particular
to ensure the commitment to a full integrated year of observation with initial
LIGO instruments.
- Allow a complete R&D cycle, with extensive testing of final designs, before
committing to fabrication.
- Bring the Advanced LIGO instruments on-line as quickly as possible to meet
the demands of the community for the observing capability of the Advanced LIGO
detector.
The schedule is based on our extensive experience with the design, fabrication, construction,
and commissioning of the Initial LIGO detectors.
The highlights of the schedule are
- Receipt of funding for the fabrication and construction project in 2008.
- Delivery of first interferometer hardware to the observatory staging facilities
in 2009.
- Decommissioning of initial LIGO at the LIGO Livingston Observatory in early 2011,
and simultaneous start of installation of Advanced LIGO there.
- Decommissioning of initial LIGO at the LIGO Hanford Observatory in late 2011,
and simultaneous start of installation of Advanced LIGO there.
- Both observatories in commissioning by mid-2013.
- Both observatories in operation by 2013.
The joint United Kingdom/German
GEO Project has received
funding
to provide a capital investment in this construction project.
They are applying these resources to providing the
suspension subsystem, including suspension assemblies, their controls, and installation
and commissioning. The German proposal is also making a capital contribution
to Advanced LIGO. It is for the design and fabrication of the pre-stabilized laser
subsystem.
The GEO Project is a full partner in Advanced LIGO, participating at all levels
in the effort.
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